Many reports have been made on technologies for raising the Young's modulus. Most of those have pertained to technologies for increasing the Young's modulus in the rolling direction (RD) and in the transverse direction (TD) perpendicular to the rolling direction (RD).
Patent Documents 1 through 9, for example, each discloses a technology for increasing the Young's modulus in the TD direction by carrying out pressure rolling in the α+γ2 phase region.
Patent Document 10 discloses a technology for increasing the Young's modulus in the TD direction by subjecting the surface layer to pressure rolling in a temperature of less than the Ar3 transformation temperature.
On the other hand, technologies for increasing the Young's modulus in the transverse direction and simultaneously increasing the Young's modulus in the rolling direction also have been proposed. That is, Patent Document 11 proposes increasing both Young's moduli by carrying out rolling in a fixed direction as well as rolling in the transverse direction perpendicular to this direction. However, changing the rolling direction during the continuous hot-rolling processing of a thin-sheet noticeably compromises the productivity, and thus this is not practical.
Patent Document 12 discloses a technology related to cold-rolled steel sheets with a high Young's modulus, but in this case as well, the Young's modulus in the TD direction is high but the Young's modulus in the PD direction is not high.
Also, Patent Document 4 discloses a technology for increasing the Young's modulus by adding a composite of Mo, Nb, and B, but because the hot rolling conditions are completely different, the Young's Modulus in the TD direction is high but the Young's modulus in the RD direction is not high.
As illustrated above, although conventionally steel sheets having “high Young's modulus” have existed, all of these were steel sheets with high Young's moduli in the rolling direction (RD) and the transverse direction (TD). Incidentally, the maximum width of a steel sheet is about 2 m, and thus, if the direction with the largest Young's modulus is the lengthwise direction of the member, then the steel sheet could not be any longer than it is wide. Consequently, a demand has existed for steel sheets with a high Young's modulus in the rolling direction, that can serve as long members. Further, hot rolling in the α+γ region, in which fluctuations in the rolling reaction force readily occur, has been a prerequisite for the manufacturing methods, and this has caused a problem in the productivity.
When processing steel sheets into components for automobiles or construction, the ability of the steel sheet to fix into the proper shape is a major issue. For example, a steel sheet that has been bent tries to spring back to its original have when the load is removed, and this may lead to the problem that a desired shape cannot be obtained. This problem has become even more pronounced as steel sheets have become stronger, and is an obstacle when high-strength steel sheets are to be adopted as components.
Patent Document 1: Japanese Unexamined Patent Application, First Publication No. S5.9-83721
Patent Document 2: Japanese Unexamined Patent Application, First Publication No. H5-263191
Patent Document 3: Japanese Unexamined Patent Application, First Publication No. H8-283842
Patent Document 4: Japanese Unexamined Patent Application, First Publication No, H8-311541
Patent Document 5: Japanese Unexamined Patent Application, First Publication No. H9-53118
Patent Document 6: Japanese Unexamined Patent Application, First Publication No. H4-136120
Patent Document 77 Japanese Unexamined Patent Application, First Publication No. H4-141519
Patent Document 8: Japanese Unexamined Patent Application, First Publication No. H4-147916
Patent Document 9: Japanese Unexamined Patent Application, First Publication No. H4-293719
Patent Document 10: Japanese Unexamined Patent Application, First. Publication No. H4-143216
Patent Document 11: Japanese Unexamined Patent Application, First Publication No. H4-147917
Patent Document 12: Japanese Unexamined Patent Application, First Publication No. H5-255804